1. Field of the invention
The present invention relates to a rechargeable power source apparatus which is capable of controlling charge and discharge of a secondary battery on the basis of ON/OFF of a switching circuit.
2. Description of the Related Art
As for a conventional charged power source apparatus including a secondary battery, there is known a charge and discharge controlling circuit as shown in a circuit block diagram of FIG. 2.
For example, such a configuration is disclosed in Japanese Patent Application Laid open No. Hei 4-75430 entitled xe2x80x9crechargeable power unitxe2x80x9d. That is, a secondary battery 101 is connected between an external terminal xe2x88x92Vo and an external terminal +Vo through a switching circuit 103. In addition, a charge and discharge controlling circuit 110 is connected in parallel with the secondary battery 110. This charge and discharge controlling circuit 110 has the function of detecting a voltage of the secondary battery 101.
When the secondary battery 101 is in the overcharge state (it means the state in which the voltage of the battery is higher than a predetermined voltage values hereinafter, this state is referred to as xe2x80x9cthe over-charge protection statexe2x80x9d, when applicable), a signal is outputted from the charge and discharge controlling circuit 110 in such a way as to turn OFF a charging switch 112 of the switching circuit 103.
On the other hand, when the secondary battery 101 is in the overdischarge state (it means the state in which the voltage of the battery is lower than a predetermined voltage value: hereinafter, this state is referred to as xe2x80x9cthe overdischarge protection statexe2x80x9d, when applicable), a signal is outputted from the charge and discharge controlling circuit 110 in such a way as to turn OFF a discharging switch 111 of the switching circuit 103. In addition, a charge current and a discharge current can be limited in such a way that the charge current or the discharge current is caused to flow through the switching circuit 103 having a resistance component, whereby the generated voltage of the external terminal xe2x88x92Vo is monitored to detect that this voltage has reached a certain voltage to turn OFF the switching circuit 103.
In other words, it is possible that when an excessive charge current is caused to flow, the charging operation is stopped (overcharge current control) and when an excessive discharge current is caused to flow, the discharging operation is stopped (overdischarge current control). Hereinafter, those states are respectively referred to as xe2x80x9cthe overcharge current protection statexe2x80x9d and xe2x80x9cthe overdischarge current protection statexe2x80x9d when applicable.
The charge and discharge controlling circuit operates to protect the battery from those states as described above.
In the case of the charge and discharge controlling circuit shown as the conventional example in FIG. 2, there arises the problem in that it is impossible to carry out the detection of the charge current with the overdischarge protection state being held, and the detection of the discharge current with the overcharge protection state being held.
Now, the problem in that it is impossible to carry out the detection of the charge current in the overdischarge protection state will hereinbelow be described specifically. Though in the overdischarge protection state, the discharging switch 111 shown in FIG. 2 is in the OFF state, in the case where a battery charger 105 is connected between the external terminal +Vo and the external terminal xe2x88x92Vo in this state, the charge current is caused to flow through the secondary battery 101, the discharging switch 111 and the charging switch 112.
However, since the discharging switch 111 is in the OFF state, the charge current is forcedly caused to flow through a parasitic diode of the discharging switch 111 (shown in FIG. 2 as a diode connected in parallel with the charging switch 111). As a result, the voltage at the external terminal xe2x88x92Vo is dropped by the voltage Vf across the parasitic diode, and in this state, it is impossible to accurately detect the overdischarge current.
The problem in that it is impossible to carry out the detection of the discharge current in the overcharge protection state is also similar to the impossibility of the detection of the charge current in the overdischarge protection state.
In the light of the foregoing, the present invention has been made in order to solve the above-mentioned problems associated with the prior art, and it is therefore an object of the present invention to make it possible to carry out the detection of the overcharge current in the overdischarge state and the detection of the overcharge current in the overcharge state by detecting with respect to the charge current, the voltage developed across a resistance component of the charging switch 112 and by detecting, with respect to the discharge current, the voltage developed across a resistance component of the discharging switch 111.